Pretty much everyone thinks of it as Calais, but the terminal's actually in Coquelles, and the tunnel emerges in Sangatte. (Yes, that Sangatte. You occasionally see asylum seekers wandering around the roads, but the police tend to keep them in slum camps or behind barbed wire, except when someone decides to have a crackdown and they bulldoze the camps, so they have to set up a new slum somewhere else.)
When you come along the A16 motorway, you just follow the signs for Tunnel sous la Manche and (provided that you don't turn right into the freight section) you end up at check in. This is pretty much identical to the UK Version. As in the UK you can buy a ticket if you haven't already done so, but most of the checkin kiosks are unmanned, as most people have already bought online.
The big difference comes next - French Customs almost always stops you and usually does a security check for explosives and firearms. They dab at your door handles and steering wheel with a little sponge on a stick, then take it away to be checked. I presume they are looking for the chemicals in explosives - and if they find any on your car, they may not be happy bunnies about letting you through. So if you are a quarryman, better be able to prove it.
Then through UK Border Control. This is a dramatic contrast to the French Border Control as you leave the UK, who simply wave you through. Here's where the long queues are. Everyone is stopped, every passport is checked. You have to drive through a big yellow Cyclamen machine - I think it detects radioactive materials by zapping you with muons...
Better allow a good half hour for the queue and the zapping and checking. They open extra lanes at peak times such as Bank Holidays, but of course at those peak times there are extra cars so you are never going to speed through this bit. The Borders Agency takes enough stick - they don't want to be pissed on by the Daily Mail.
The terminal itself is fairly similar to the UK side, except that the duty free section is considerably larger. Makes sense - nobody is going to be taking booze from the UK to France, except if you want to give your neighbour an unusual bottle of malt whisky, perhaps. If you haven't already filled up at the supermarket (there's a big Auchan in Sangatte, closed on Sunday though) you'll find a reasonable choice here. There's a Quick (French hamburger joint) and a W H Smiths where you can pay outrageous prices for imported UK newspapers to read on the train.
And there's a (spit) newly opened Starbucks, which replaced the French café chain Brioche Dorée about a month ago. I learned the other day that Starbucks espresso tastes burnt because that is the way folks in Seattle like it - I suppose there is no accounting for tastes. Fortunately there are coffee machines as you queue to get on the trains, which also let you get rid of your small change...